The present invention relates to optical devices wherein the image seen by a viewer depends on the viewer""s angle of regard with respect to the plane of the windowpane-shaped device. More particularly, the invention relates to an extraordinarily thin lenticular sheet with periodic optical elements formed in such a way that the viewer will perceive a series of images that change depending on the viewer""s angle of regard. The invention presents an improved manufacturing technique for such sheets that overcome the unsatisfactory results achieved in dimensional tolerance and stability with current methods applied to fine dimensions.
The concern of this patent is very thin lenticular sheets wherein the finished product is of comparable overall thickness to 10 to 24-lb bond paper. Thus, the product can be used for magazine blow-ins or pages in magazines that use perfect bindings. It can be used in many other special applications that have heretofore been unattainable due the relatively stiff nature of the lenticular product. For example, a sufficiently thin lenticular film can be used to decorate a tee-shirt, or be incorporated any imaginable place where paper-thin products might be used.
A lens array is fabricated by first designing a cutting tool with the desired lens shape, then using this tool to cut the negative of the shape of the array of lenses into a cylindrical roll. A lenticular plastic sheet is usually produced by the extrusion of a single layer from a thermoplastic resin melt into a continuous web. The extrusion equipment consists of a die, followed by a roll stack whereby the lens pattern on the cylinder is pressed into the viscous resin using an embossing roll with counter pressure being provided by a nip roll. In manufacturing plants today, the lens array is almost always produced by molding the appropriate shape into the surface. Cross-linking of the thermoplastic is enabled thermally in the extrusion process. Cooling the thermoplastic lenticular resin sets the relief pattern on the surface. In some cases, actinic radiation, usually in the form of ultraviolet light of appropriate wavelength and radiance is used to cross link said resin and thereby set the relief pattern.
To get said thermoplastic web in condition to be divided into sheets or readied for shipment on a take-spool, pull-rolls are used to draw the substantially cooled and set material. These rolls unavoidably introduce longitudinal tension into the web. Longitudinal tension is tension in the machine direction. This tensioning sometimes causes significant stretching and necking of a thin web causing the lenticular surface to distort. The distortion is not uniform. The lens pattern departs from design increasingly from the center of the web outward. The distortion is only approximately predictable. The tension can vary. Dimensions can shift when the web is divided into sheets. Temperature variability induces distortion variability. Some newly formed thermoplastics are hygroscopic and ambient humidity induces distortion variability. Attempts at introducing windage based on distortion data from a specific extrusion plant and material ameliorate the distortion somewhat. However, this solution is increasingly inadequate for increasingly thin end material. For the objectives of this patent, these effects are paramount. For thin sheets (0.012-inches to 0.002-inches thick), the aforementioned dimensional stability problems can result in an unacceptable product. The required linear registration can be problematic or impossible.
It is important to put the lenticular sheet in context of the end product. A computer-generated image is printed on the planar side of the product. The printing can be direct or a print can be prepared in a separate operation and laminated to the lenticular sheet. Either way, the print is registered and precisely aligned to the front-side lenticular array that consists of a set of parallel, longitudinal, cylindrical elements that are substantially circular in cross section. If the lenses run up and down, three-dimensional images, that is, multiple images at apparently different distances from the eyes in space, can be designed. These images are not continuous like holograms, but are convincing. For example, one image may occlude another on a xe2x80x9cdeeperxe2x80x9d plane. The viewer can shift their angle of regard right or left and the previously occluded image will appear. If the lenses are horizontal, other interesting optical effects called flip, morph, motion, zoom and others are possible. Typically, a repertoire of two to eight images is dissected and affixed in a precisely aligned and registered fashion to said lenticular array.
If extremely thin products were not a concern, the image may be affixed in a number of ways. A registration mark is molded into the lenticular array. Since the array consists of linear parallel lenses, only one alignment mark is needed, the array itself provides the angular reference from that mark. The image can be printed on paper by almost any high-resolution printing technique. The paper can be aligned to the previously mentioned mark and laminated to lenticular away. Since our final product thickness is comparable to a sheet of paper, this technique is not available. The precision of the printing is critical. The array thickness is related to the lens spacing. A rule-of-thumb is that the overall thickness must be the lens spacing, D, times n/2n(nxe2x88x921) where n is the index of refraction of the material. Since the instant invention seek sicknesses on the order of 0.002-inches, the lens spacing is unavoidably on the order of 0.005-inches. The printer image technology must be at least five times more accurate than the lens spacing, on the order of 1000 lines per inch or more. The alignment must be on the order of 0.0005-inches and the orthogonality within a small fraction of a degree. Several suitable technologies are based on photolithographic printing and thermal or piezoelectric ink jet printing which are conventional except for the alignment means. With advances in photolithographic techniques driven by the semiconductor industry""s pursuit of Moore""s law, it is and likely will continue to be the highest resolution printing technique. The actual implementation of photolithographic printing may involve precision printing on a transfer roll and transfer printing on the end product. Despite assertions to the contrary, intaglio printing such as gravure printing wherein Cyan, Yellow, Magenta and Black inks are placed in a series of steps and thus require four precision alignments and registrations are generally applicable to lower resolution and therefore thicker embodiments of lenticular products. Due to the resolution of the eye, there is little discernable difference between 600 and 1200 pixels per inch when viewed at 15 to 18-inches. However, each repertoire image must exist at each lens crossing. To achieve eight images at an effective resolution of 600 pixels per inch requires 4800 sub pixels per inch, since the lens selects only an eighth of the underlying print at any specific angle of regard. The print resolution is therefore determined by the thickness objectives.
We are aware of the following related art:
The teachings of U.S. Pat. No. 4,420,502 (""502) differ from the currently used manufacturing technique in that a base film that is intended to be dimensionally invariant covers the nip roll while a lenticular resin is extruded between the molding roll and the nip roll. In a first embodiment, the base film is incorporated into the product, thus would either receive print directly or have an image that is printed on paper bonded to it. The base film incorporates adhesion promoters to assure a good bond between the lenticular resin and the base film. In a second embodiment, the adhesion promoter has a different characteristic, a release additive, such that once cooled the base film may be stripped from the lenticular film. The invention solves the problem of lamination in higher resolution applications. The lenses must focus at the image surface. In unimproved materials, the tolerances sometimes accumulate during lamination reducing the quality of the image. Since the base film and the lenticular film are subjected to formation concurrently between the nip roll and the engraved molding roll, dimensional variation is substantially keyed to the dimensional stability of the base film. The base film can be coated with a light-sensitive emulsion and the image created photographically. The base film may be polyethylene terephthalate, PET. The lenticular resin may be actinically radiation-cured. The required bilateral stability required for very thin arrays is not addressed in this patent. The base film might be a PET film without the necessary cross linking to protect a very thin lenticular resin from distortion. 
The above is the PET monomer. The dimensional variation referred to in ""502 is thickness variation. The concern of thin lenticular arrays is longitudinal and latitudinal variation that results in an array that cannot be registered and aligned. Elements of ""502 exist in the instant invention, However, the instant invention directs improvements and presents new alternatives to ""502. In one embodiment in ""502, the base film is stripped in post-molding processing. No solution to handling thin films prior to adding the planar surface print is suggested.
In U.S. Pat. No. 4,420,527 (""527), the claims are directed to the resulting product, rather than the method of manufacture. The product is produced similarly to ""502. A base film that may be PET is used with an adhesion promoter. The pliant lenticular resin is molded by roll with the negative of the desired relief pattern. The base film covers the nip roll during molding. Actinic radiation is used to cross link the lenticular polymer. A photographic emulsion layer is incorporated.
In U.S. Pat. No. 4,414,316 (""316), the claims in ""527 are extended to include features taught in ""527.
U.S. Pat. No. 5,362,351 (""351) does not mention the above inventions as prior art. Nonetheless, ""351 uses and claims a base film with an adhesion layer and the extrusion of the lenticular resin for molding against an engraved roll with counter pressure being applied via a nip roll. The adhesion layer may be applied in a prior step or co-extruded. However, PET film suppliers will supply their films with an adhesive layer if desired. In both, ""351 and in ""502, the option of separating the molded lenticular film from the base film is presented. There is an assertion in ""351 that tandem extrusion lines are frequently employed resulting in inclusion of air bubbles or the like due to the heating and cooling of the product. It is further asserted that biaxially stabilized PET base films are not satisfactory because they are costly. Accordingly, ""351 teaches non-oriented polypropylene and non-oriented PET films as base films. It is asserted in ""351 that stronger adhesives are possible because the adhesive does not contact chilled rolls that can include the engraved molding roll, the nip roll and subsequent rolls. While true, this is hardly an innovation of ""351 since the same attributes exist in the above cited inventions. The printing technology of choice in ""351 is gravure printing. It is asserted that intaglio printing of this kind is successful because the molding roll and subsequent gravure printing rolls are precisely aligned because they are manufactured by the same process. However, the results claimed are inconsistent with the objectives of this invention.
U.S. Pat. No. 4,042,569, Heat-setting process for polyester film, can be summarized as an improvement in a two-stage heat-setting process for asymmetrically, biaxially oriented polyester film wherein the film from the second heat-setting stage is heat set a third time and quenched. The starting film can be oriented by a transverse-direction then machine-direction sequence of drawing. Alternatively, the starting film can be oriented by a machine-direction, transverse-direction, and finally, machine-direction sequence of drawing. The film can be machine direction drawn again after the second heat-set stage and relaxed an equal amount after the third heat-set stage. Said film has improved dimensional stability in the transverse direction without a substantial loss in tensile strength at 5% elongation in the machine direction. Starting films with the cited properties are critical to the accomplishments of my invention.
The objective of my invention is to achieve exceptionally thin optically anisotropic lenticular sheets for the realization of anisotropic visual effects such as three dimensional effects for vertically oriented lenses"" axes and the gamut of effects for horizontally oriented lenses"" axes such as morph, motion, zoom and others. These new thinner embodiments are consistent with the advances in printing technology that can align and register planar surface print with sufficient detail to present a repertoire of as many as eight images in these extraordinarily thin realizations.
This objective is achieved by an improvement of previous art. The departure from the method of previous inventions consists of the use of a stabilized carrier film. In order to defeat the distortion created by the longitudinal tension effected by the pull rolls, a biaxially oriented heat stabilized carrier film is introduced at one of several appropriate places in the web process. This film is typically from the polyethylene terephthalate, PET family of materials and has been heat treated at temperatures that exceed any temperature encountered in the web process.
In some embodiments, the stabilized base film stays with the product. In these embodiments, said film may be coated at the film manufacturers to adhere to the extruded lenticular resin. It is sometimes convenient to simultaneously extrude a tin adhesion layer on the biaxially stabilized but untreated carrier film. Precision, aligned ink jet printing requires a print surface with the appropriate absorption characteristics for the rheology of the specific inks employed. The surface energy of this back layer is critical. If the surface energy is too high, the ink droplets will form globules that fail penetrate the surface resulting in a smear. If the surface energy is too low, the ink droplets will adsorb and spread out in a dendritic pattern, ruining the inherent resolution capability of the print. Another important characteristic of this back layer is high diffuse reflectivity. The inks are responsible for the color absorption. For example, inks usually consist of yellow (which absorbs blue incident light), cyan (which absorbs red incident light) and magenta (which absorbs green incident light), and sometimes black (which absorbs all wavelengths). The luminance of the reflected light comes only from the visibility-weighted diffuse reflectivity of the back layer. It should be color neutral, white. The critical planar-side ink-receiving layer to match the intended ink can be procured with the base film or introduced in the web process.
In some embodiments, the image is transfer printed on the planar side. With transfer printing, the planar side may be prepared differently. The surface energy of the planar-side film may be treated with a corona to condition the surface to this type of image technology.
In very thin embodiments, it is sometimes impossible to retain the carrier film with the product. In such cases, the adhesion layer is modified to allow the lenticular film separation. Since these are very thin lenticular sheets, it is sometimes necessary to add a handling film to the lens side of the lenticular sheet for handling. After the film is printed, the sheet may be peeled away for viewing of the final product.
In some embodiments, the required relief pattern on the viewer-side of the lenticular array is not moulded by extruding the lenticular resin ahead of the roll that has been engraved with the negative or the desired surface texture. Instead, a novel multilayer film is introduced into web process. The film has two critical layers, a biaxially stabilized, relatively high temperature film and a lenticular resin. The resin is so designed that the required relief pattern can be effected by heating the multilayer film and embossing the multilayer film with an engraved roll. The composite film is heated to effect cross linking of the lenticular polymer. After molding, heat is removed from the composite film to set the relief pattern. The manufacturing step of extruding a lenticular resin is absent in this embodiment. If inkjet printing is desired, an ink-receptive layer may be added. Alternatively, the planar surface may be conditioned with a corona discharge for other imaging techniques.
In further embodiments, the longitudinal stress in the film is further managed by sensing a force that is linearly related to said stress and incorporating the sensor output in a servomechanism to minimize and control said stress. In any of the embodiments described herein, the print receptive layer that may be bonded to the planar side of the film may sometimes be a preprinted film or preprinted paper that is simultaneously bond to said lenticular sheet. In the case of bonding of a preprinted film or sheet, the product may be completed in a single web process.
The principal object of the invention is to provide a windowpane-shaped device that presents the repertoire of images differently to the right and left eye of the viewer to yield the perception of depth, or to present the standard optical effects of morph, motion, flip or zoom with previously unattainably lower thickness.
Another object of the invention is to provide a method to manufacture previously unattainably thin lenticular sheets with sufficient dimensional stability to register a computer-designed image to effect these optical perceptions.